This invention relates generally to turbine rotor assemblies and, more particularly, to seal systems for sealing radial leakage in turbine bucket covers.
Known steam turbines are classified as action turbines, or “constant-pressure” turbines and reaction turbines, or “excess-pressure” turbines. Each turbine includes a turbine shaft that includes moving blades, or buckets positioned circumferentially around the shaft, and includes an inner casing with guide blades, or diaphragms positioned between axially spaced buckets.
In the case of a constant-pressure turbine, the entire energy gradient is converted essentially into kinetic flow energy in the ducts that are narrowed by the guide blades. During the process, the velocity rises and the pressure falls. In the moving blades, the pressure and relative velocity remain essentially constant, being achieved through ducts having a uniform clear width. Because the direction of the relative velocity changes, action forces occur that drive the moving blades and, thus, cause rotation of the turbine shaft. The magnitude of the absolute velocity decreases considerably when the flow passes around the moving blades, resulting in a flow that transfers a large part of its kinetic energy to the moving blades and, therefore, to the turbine shaft.
In the case of an excess-pressure turbine, only part of the energy gradient is converted into kinetic energy when the flow passes through the guide blades. The rest of the energy gradient brings about an increase in relative velocity within the moving-blade ducts formed between the moving blades. Where the blade forces are almost exclusively action forces in the constant-pressure turbine, in an excess-pressure turbine, a greater or lesser fraction resulting from the change in the velocity magnitude is added. The term “excess-pressure” turbine is derived from the pressure difference between the downstream and the upstream side of the moving blade. In an excess-pressure turbine, therefore, a change in the velocity magnitude takes place when the pressure varies.
A turbine typically includes at least one rotor including a plurality of rotor buckets or blades that extend radially outwardly from a plurality of wheels attached to a common annular shaft. Specifically, the rotor buckets are attached to the wheels with dovetail joints. A radial extrema of the bucket, or tip may support a cover that joins the tips of a plurality of buckets circumferentially around the periphery of the turbine. In some known bucket designs the cover is individual to each bucket and integrally cast with the bucket during manufacture.
A gap between adjacent bucket covers may define a radial leakage path that may allow the working fluid to escape the bucket surfaces and adversely affect an operational efficiency of the turbine.
To facilitate reducing radial leakage, rotor assemblies may include sealing covers. At least some known rotor assemblies include additional loose fitting seals that are separate from the bucket covers. The seals block the leakage path to reduce radial leakage.
During operation, forces induced within the bucket may cause the adjacent bucket covers to flex and to separate from each other increasing the width of the leakage gap. Over time, continued leakage between the adjacent bucket covers may erode the covers where adjacent covers adjoin and increase leakage flow, adversely affecting the turbine operational efficiency.